The present invention relates generally to medical dispensing devices and, more particularly, to an injector pen that permits selectively measured doses to be dispensed.
Patients suffering from diseases such as diabetes must inject themselves several times each day with an insulin solution. Since the volume of insulin solution to be injected varies from injection to injection, it is necessary for such patients to be able to measure a precise volume of insulin. Diabetics have conventionally used a typical syringe for injection of insulin. However, it is difficult to control the operation of the syringe as well as the quantity of drug injected.
In order to permit a diabetic to measure and administer a more accurate and controlled dosage, injector pens have been developed, which enable a particular dosage to be accurately and conveniently measured. Generally, these pens are threaded onto a cartridge having a particular quantity of liquid medication sealed therein. The cartridge includes a plunger and a mechanism for advancing the plunger in the cartridge in such a manner to dispense the medication.
One such pen that has functioned very adequately is the OptiPen.RTM., which is manufactured by Disetronic AG, of Burgdorf, Switzerland, and which is commercially available through Hoechst of Frankfurt, Germany. This pen includes an LCD assembly for conveniently "dialing up" the desired dosage. To use this pen, a release button is actuated in order to unlock the dosage knob and energize the LCD. The old insulin cartridge is removed and a new insulin cartridge is threaded onto the injector housing. The dosage knob is then turned, which sequentially increases the number appearing in the LCD until the desired dosage setting appears on the LCD. Each number on the display represents a predetermined dosage volume, and each turn of the dosage knob causes an audible "click" sound to be heard. Each turn of the dosage knob causes a threaded rod to move forward in the cartridge. Once the desired dosage is obtained, the needle attached to the end of the cartridge is injected into the skin, and the dosage knob is pressed in slowly until it clicks into place, assuring that the desired dosage of insulin has been injected.
Another dispensing assembly is disclosed in U.S. Pat. No. 4,936,833, issued to Sams. This dispenser includes a disengageable drive mechanism, such as a pair of jaws, that is controlled by a rotatable section of the housing that houses the engagement mechanism This "sleeve", when rotated, disengages the pair of jaws from the plunger rod to permit the rod to be moved into its "zero" position. The rod is then set to the desired dosage, causing a push button to become extended. The drive mechanism is then re-engaged with the rod, and the push button is depressed to administer the dose of insulin.
In addition to delivering insulin, mechanical injectors are useful for the delivery of human growth hormone which, like insulin, must be administered periodically. A problem with the OptiPen described above is that it does not permit a user to correct the error of inadvertently dialing up too great an amount of medication to be dispensed. Any resetting of the dosage of this pen would require that the cartridge be removed from the injector, which not only necessitates considerable effort, but more importantly, generally results in inaccurate dosing, especially if a significant dosage has been initially overdialed. As a result, if too great a dosage is dialed up, it is generally necessary to discharge the entire dose and start over. This is quite problematic in the delivery of growth hormone, since growth hormone is tremendously expensive. Accordingly, it is desirable to provide a dispensing device that permits a user to accurately and conveniently correct an overdial situation.